1. Field of the Invention
The present invention concerns an antenna for portable radio devices and in particular for mobile telephones, a method of manufacturing an antenna of this kind and a portable radio device including an antenna of this kind. The antenna is used to transmit and to receive radio signals.
2. Description of the Prior Art
The antennas currently used in portable radio devices, and more particularly in mobile telephones, usually include:
a quarter-wave helical antenna in the upper part of the telephone casing and fed by a coaxial line coupled to the transmitter/receiver of the radio device for use under normal conditions, the helix usually comprising a metal wire wound around an insulative material former, PA1 optionally, for using the radio device in the presence of strong interference, a half-wave whip retractable from the casing of the radio device and such that when it is retracted inside the casing it is virtually entirely decoupled from the helical antenna and when it is deployed from the casing it is capacitively coupled to the helical antenna.
Antennas of this kind are described in patent application EP-A-0 367 609 and in U.S. Pat. No. 4,121,218.
Although the radio performance of such antennas is acceptable for this application it is not optimized, in particular in terms of bandwidth and efficiency. This is because their radio impedance, characteristic of their radiating power and consequently of their efficiency as antennas, is low (in practise very much lower than 50 .OMEGA.).
Also, given the currently relatively small size of portable mobile telephones, it is desirable to make the retractable whip as small as possible so as to take up as little space as possible inside the casing of the telephone, in which it is housed when retracted. The volume occupied inside the casing by the retractable whip cannot be occupied by other components necessary for the operation of the mobile telephone (transmitter/receiver, modulator/demodulator, encoder/decoder, smartcard connector, etc).
Prior art retractable whips are usually substantially cylindrical and as a result occupy too great a volume within the mobile telephone casing.
Further, the performance of this prior art type antenna using a retractable whip in terms of gain and omnidirectionality is degraded by the interdependence of the retractable whip and the mobile telephone casing.
Consequently, one object of the present invention is to provide an antenna for portable radio devices of increased efficiency as compared with prior art antennas of this type.
Another object of the present invention is to provide an antenna of the above type which occupies the smallest possible volume within the portable device.
A further object of the present invention is to provide an antenna of the above type in which the retractable whip is as independent as possible of the casing of the associated radio device.